We're still new to the game, and we have no idea what we're looking for.

I think it's incompetence.

seriously though, I think life on other planets probably just usually evolves underground, so even if they develop some sort of intelligence they're not looking up at the sky so they have no motivation to explore beyond their atmosphere no matter how advanced they get.

there was a planet in The hitchhiker's Guide to the Galaxy universe that had thick cloud cover so that people never even conceived of an existence beyond their planet. when a spaceship crashed there, it never even occurred to them that it might have come from the sky

I always thought of it as a series of tests or filters. Like a multistage filter. So like nukes is one, responsible environment management is another. Something like photosynthesis is more of a conditions for life to emerge thing to me really. If like can flourish to begin with then mutations are common enough that things like photosynthesis are inevitable.

Energy needed to leave your planetary system vs energy available on your planet of origin.

We have not yet overcome it and I am not sure that we will achieve it.

I don't think there is a great filter. I think there's an easy solution to the fermi paradox that doesn't require great filters, we're just the first intelligence in this galaxy.

Here's my reasoning: intelligent species that manage to develop space travel probably do tend to expand out into their galaxy. When they achieve this level of technology they can settle most of all of their galaxy in a matter of 10,000 years or so. That time period is very brief on an evolutionary scale. It's estimated that life began on earth 3.7 billion years ago. That means it took about 3.7 billion years for earth to produce intelligent life, and then from that point it would take a mere 10,000 years to reach modern day, and 10,000 more years to settle the whole galaxy. That expansion happens so quickly compared to how long it took the planet to develop intelligent life, that the chance of two civilizations rising at the same time becomes very small.

It all boils down to this: there are no intelligent aliens out there in our galaxy, because we are the first intelligent species in our galaxy. We know we're the first because if we were second, then aliens would already have settled this star system.

Probably there are lots of alien civilizations out there in the universe, but they're in different galaxies.

It's very very obviously the speed of light.

Space itself. I believe there are other intelligent life forms out there and some of those happen to be close enough to communicate to each other/discover each other. We just hit the unlucky(or lucky) spot that we are simply too far away.

evidence of human existence has only gotten what, 150 lightyears out into space?

Either multicellular life or that societies that are bent on expansion at any cost tend to destroy their planet's ecosystem before they can establish themselves outside of it. Not making a definitive claim on either, obvs. We have an extremely low sample size after all.

I think it would be nuclear warfare. Nuclear fission is a universal development for any advanced civilization. It would be easy to construct a nuclear bomb in an advanced civilization. Once a few rogue/pariah states start making them, everyone's screwed.

Making nukes is easy, the only reason we don't see more nuclear states on earth is because of the international backlash. With a couple more Iran and North Korea's we'll likely meet the filter ourselves.

Personally? Nationalism & nation states. The longer they stay around, the more likely everyone is to think they're more deserving of X, and pull the literal and metaphorical trigger that leads to hitting the filter.

I recognize that individuality is very much our thing, but that will literally only take you so far.